OPTICAL ISOMER SEPARATING AGENT AND PROCESS FOR PRODUCING THE SAME

Abstract: A polysaccharide derivative is immobilized on a carrier such as silicagel by cross-linking only the derivatives with each other by using a polyfunctional cross-linking agent. The optical isomer separating agent thus obtained is excellent in solvent resistance and is most suitable for use in optical resolution.

Description: The present invention relates to a separating agent for optical isomers and a process for producing the same. More particularly, it relates to a separating agent, which is obtained by crosslinking polysaccharide derivative molecules exclusively among themselves on a support and useful as an optical resolution reagent for racemic modifications, and a process for producing the same. It is known that a column packing composed of silica gel having a polysaccharide derivative supported thereon is useful as a separating agent for optical isomers of a racemic modification (Y. Okamoto, M. Kawashima and K. Hatada, J. Am. Chem. Soc., 106 , p.5357, 1984). However, such a separating agent is poor in solvent resistance since it comprises silica gel having a polysaccharide derivative which is supported on the silica gel simply by coating alone. When the separating agent is used as a column packing for liquid chromatography, therefore, some eluents cannot be used. Accordingly, a separating agent which is composed of a support having a polysaccharide derivative being supported thereon and has an excellent solvent resistance has been required. EP-A-0281951 discloses the use of optically active polysaccharide derivatives insolubilized by crosslinking for separating racemic mixtures. The present inventors have conducted extensive studies on a separating agent whereby the above-mentioned problems can be solved without deteriorating the excellent characteristics of a polysaccharide derivative and, as a result, completed the present invention. In a first aspect, the present invention provides a separating agent for optical isomers characterized by being composed of a polysaccharide derivative which has been immobilized on a support by crosslinking, on said support, the polysaccharide derivative molecules with the use of a polyfunctional crosslinking agent which does not form a link between said support and said polysaccharide derivative molecules (i.e. the polysaccharide derivative molecules are crosslinked exclusively among themselves), and wherein the surface of said support has been inactivated prior to said crosslinking reaction in order to prevent the surface of said support from reacting with said crosslinking agent. In a second aspect, the present invention provides a process for producing a separating agent for optical isomers as specified above characterized by crosslinking, on a support, polysaccharide derivative molecules with the use of a polyfunctional crosslinking agent which does not form a link between said support and said polysaccharide derivative molecules, wherein the surface of said support has been inactivated prior to said crosslinking reaction in order to prevent the surface of said support from reacting with said crosslinking agent. The present invention is also directed to the use of a separating agent for optical isomers as defined above as a column packing in chromatography. Additionally, the present invention provides a process for separating a mixture of optical isomers which comprises using the above separating agent for optical isomers to thereby separate the mixture of optical isomers from each other. Specific embodiments of the present invention are set forth in claims 2 to 5 and 7 to 10. Although the polysaccharide in the present invention may be any of a synthetic polysaccharide, a natural polysaccharide and a modified natural polysaccharide without restriction, so long as it is an optically active one, those having a high regularity in the mode of binding are preferable. Examples of the polysaccharide include β-1,4-glucan (cellulose), α-1,4-glucan (amylose, amylopectin), α-1,6-glucan (dextran), β-1,6-glucan (bustulan), β-1,3-glucan (e.g., curdlan, schizophyllan), α-1,3-glucan, β-1,2-glucan (crown gall polysaccharide), β-1,4-galactan, β-1,4-mannan, α-1,6-mannan, β-1,2-fructan (inulin), β-2,6-fructan (levan), β-1,4-xylan, β-1,3-xylan, β-1,4-chitosan, β-1,4-N-acetylchitosan (chitin), pullulan, agarose and alginic acid. In addition thereto, starches containing amylose are involved in the category of polysaccharides. Among them, those which can be easily obtained in a highly pure state, for example, cellulose, amylose, β-1,4-chitosan, chitin, β-1,4-mannan, β-1,4-xylan, inulin and curdlan, are preferable and cellulose and amylose are preferable in particular. The number-average degree of polymerization (the average number of pyranose or furanose rings per molecule) of the polysaccharide is 5 or above, preferably 10 or above. Although the upper limit of the number-average degree of polymerization of the polysaccharide is not particularly defined, it is preferable that the number-average degree of polymerization thereof does not exceed 500 from the viewpoint of workability. An example of the polysaccharide derivative to be used in the present invention is a compound which is obtained by reacting such a polysaccharide as those cited above with a compound having a functional group capable of reacting with a hydroxyl group of the polysaccharide by a publicly known method and in which some of the hydroxyl groups of the starting poly-saccharide form ester bonds or urethane bonds. Examples of the compound having a functional group capable of reacting with a hydroxyl group include isocyanic acid derivatives, carboxylic acids, esters, acid halides, acid amides, halides, epoxides, aldehydes, alcohols and other compounds having leaving groups. They may be any of aliphatic compounds, alicyclic compounds, aromatic compounds and heteroaromatic compounds. Particularly preferable polysaccharide derivatives to be subjected to the crosslinkage in the present invention are esters or carbamate derivatives of cellulose or amylose having at least 0.1 unreacted hydroxyl group per glucose unit and compounds having at least 0.1 reactive, functional group per glucose unit in the groups introduced into cellulose or amylose so as to convert it into the ester or carbamate derivative thereof. As examples of the reactive functional group, hydroxyl group, amino group, mercapto group, carboxyl group and vinyl group may be cited. Examples of the support to be used in the present invention include porous organic supports and porous inorganic supports of which surfaces have been inactivated to thereby prevent them from reacting with a crosslinking agent, and porous inorganic supports are preferable. Appropriate porous organic supports include those composed of polymers such as polystyrene, polyacrylamide and polyacrylate. Appropriate porous inorganic supports include silica gel, silica, alumina, magnesia, glass, kaolin, titanium oxide and silicates. As a particularly preferable support, silica gel may be cited. The particle size thereof ranges from 1 µm to 10 mm, preferably from 1 to 1,000 µm and still more preferably from 1 to 300 µm, while the average pore size thereof ranges from 10 Å to 100 µm, preferably from 50 to 50,000 Å. The treatment for inactivating the surface of silica gel can be effected by a publicly known method. Examples of the polyfunctional crosslinking agent to be used in the present invention in order to crosslink polysaccharide derivative molecules exclusively among themselves include polyfunctional isocyanate derivatives, acid chloride derivatives of dicarboxylic acids, diepoxy derivatives and divinyl derivatives. These crosslinking agents may be any of aliphatic compounds and aromatic compounds. As the polyfunctional crosslinking agent, diisocyanate derivatives are preferable in particular. In the present invention, the one composed of a crosslinked polysaccharide derivative which is obtained by selectively crosslinking the hydroxyl groups at the 6-position of cellulose or amylose with each other is particularly preferable. The crosslinking ratio of the polysaccharide derivative preferably ranges from 1 to 20%. The term "crosslinking ratio" as used herein means the ratio of the unreacted hydroxyl groups and/or reactive functional groups participating in the crosslinkage to all the hydroxyl groups of the starting poly-saccharide, supposing that the unreacted hydroxyl groups or reactive functional groups in the polysaccharide derivative react with the functional groups of the polyfunctional crosslinking agent at a ratio of 1 : 1. To crosslink the hydroxyl groups in the polysaccharide derivative with each other, the reactive functional groups therein with each other or the hydroxyl groups with the reactive functional groups therein, the polysaccharide derivative must be supported (immobilized) on a support the surface of which has been previously subjected to inactivative treatment. An example of the method for immobilization is coating of the support with a solution of the polysaccharide derivative. The amount of the polysaccharide derivative to be immobilized on the support ranges preferably from 1 to 100% by weight, still more preferably from 5 to 60% by weight, based on the weight of the support. Now an example of the process for producing the separating agent of the present invention will be described. Cellulose is reacted with trityl chloride to thereby give 6-O-tritylcellulose. The tritylcellulose thus obtained is reacted with a compound which has a functional group capable of reacting with a hydroxyl group by a publicly known method. Thus the hydroxy groups in the tritylcellulose form ester bonds or urethane bonds together with the functional groups of the compound having the functional group capable of reacting with a hydroxyl group. The cellulose derivative thus obtained is treated with an acid such as hydrochloric acid to thereby eliminate its trityl groups. Thus a cellulose derivative used according to the present invention is obtained. The obtained cellulose derivative is then dissolved in a solvent. Silica gel the surface of which has been inactivated is coated with the obtained solution to thereby give a silica gel coated with the cellulose derivative. The cellulose derivative of the cellulose-derivative-coated silica gel is reacted with a polyfunctional isocyanate derivative in a dry, inert solvent to thereby crosslink the cellulose derivative molecules among themselves. Thus the separating agent according to the present invention wherein a cellulose derivative is immobilized on silica gel can be obtained. The methods for immobilizing a polysaccharide derivative on a support by crosslinking the polysaccharide derivative molecules exclusively among themselves on the support in the present invention include a method wherein the crosslinkage is effected in the hydroxyl group moiety originating in the starting polysaccharide of the polysaccharide derivative and another method wherein the crosslinkage is effected in the reactive functional group moiety carried by the groups which have been introduced into the polysaccharide in the step of the conversion of the polysaccharide into its derivative. The former includes the methods represented by the following reaction scheme. When the crosslinkage is effected at the reactive functional group moiety carried by the groups which have been introduced into the polysaccharide in the step of converting the polysaccharide into its derivative, a step of introducing a substituent carrying the reactive functional group may be added to each of the processes represented by the above-mentioned reaction scheme. Namely, when the polysaccharide is converted into the derivative prior to the crosslinkage, a compound having a functional group capable of reacting with a hydroxyl group together with another reactive functional group may be reacted with the polysaccharide or the polysaccharide derivative wherein specific hydroxyl groups are protected. In the separating agent of the present invention obtained by such a method, a crosslinked polysaccharide derivative which is obtained by crosslinking polysaccharide derivative molecules among themselves with the use of a polyfunctional crosslinking agent includes a support therein to form such a structure that the support would not be released from the crosslinked polysaccharide derivative. The separating agent for optical isomers according to the present invention is generally used in chromatographies such as gas chromatography, liquid chromatography and thin layer chromatography. Application thereof to liquid chromatography is particularly preferable. Thus, the separating agent for optical isomers according to the present invention is used as a column packing in various chromatographies. Use of the separating agent for optical isomers of the present invention makes it possible to separate a mixture of optical isomers from each other. The separating agent of the present invention has an excellent solvent resistance and, therefore, is most suitable as a separating agent for optical resolution. Examples The present invention will be hereinafter described in detail with reference to Examples. Example 1 1 ○ Surface inactivative treatment of silica gel Porous silica gel (Daiso SP-1000) was subjected to aminopropylsilane treatment (APS treatment) by a publicly known method. In 1,000 ml of methylene chloride, 200 g of the APS-silica gel thus obtained was reacted with 15 ml of 3,5-dimethylphenyl isocyanate at room temperature for 1.5 hours. The obtained product was separated by filtration with a glass filter, washed successively with a mixture of methylene chloride with methanol (2/1), methylene chloride, ethanol, acetone and n-hexane, and dried. Thus, silica gel of which surface had been subjected to inactivative treatment was obtained. 2 ○ Synthesis of cellulose derivative [6-hydroxy-2,3-bis(3,5-dimethylphenylcarbamate) derivative of cellulose] Under a nitrogen atmosphere, 4.0 g of trityl-cellulose having 0.9 to 1 trityl group per glucose unit was dissolved in dry pyridine. To the solution thus obtained was added 10 ml of 3,5-dimethylphenyl isocyanate. Under a nitrogen atmosphere, the resulting mixture was stirred at 100°C for 25 hours under heating. The reaction mixture thus obtained was poured into 700 ml of methanol. The solid thus precipitated was separated by filtration with a glass filter, washed successively with ethanol and n-hexane, and dried. Next, this solid was introduced into methanol containing conc. hydrochloric acid. The mixture thus obtained was stirred to thereby eliminate trityl groups from the reaction product. The solid was separated by filtration with a glass filter, washed successively with ethanol and n-hexane, and dried. Thus a 6-hydroxy-2,3-bis(3,5-dimethylphenylcarbamate) derivative of cellulose was obtained. 3 ○ Preparation of silica gel supporting cellulose derivative thereon 1.5 g of the cellulose derivative obtained in the above step 2 ○ was dissolved in tetrahydrofuran. The resulting solution was uniformly poured onto 5.7 g of the silica gel obtained in the above step 1 ○ and the solvent was volatilized to thereby support the cellulose derivative on the silica gel. The product thus obtained was washed successively with methanol, ethanol and n-hexane, and dried. Thus silica gel supporting the cellulose derivative thereon was obtained. 4 ○ Immobilization of cellulose derivative on silica gel through crosslinkage exclusively among cellulose derivative molecules To 6.7 g of the silica gel supporting the cellulose derivative thereon obtained in the above step 3 ○ was added 35 ml of toluene which had been dried over metallic sodium (hereinafter referred to dry toluene). Further, 110 mg of diphenylmethane diisocyanate was added thereto. The mixture thus obtained was stirred at 110°C for 6 hours under heating. After the completion of the reaction, the product was separated by filtration with a glass filter, washed successively with tetrahydrofuran, methanol, ethanol and n-hexane, and then dried. Thus a separating agent composed of the cellulose derivative immobilized on the silica gel was obtained. 5 ○ Modification of unreacted hydroxyl group in cellulose derivative immobilized on silica gel To the separating agent obtained in the above step 4 ○ were added 25 ml of dry toluene and 15 ml of dry pyridine. Further, 0.5 ml of 3,5-dimethylphenyl isocyanate was added thereto. The resulting mixture was stirred at 110°C for 15 hours under heating. After the completion of the reaction, the product was separated by filtration with a glass filter, washed successively with tetrahydrofuran, methanol, ethanol and n-hexane, and dried. Thus the unreacted hydroxyl groups in the cellulose derivative immobilized on the silica gel were carbamoylated. The amount of the cellulose derivative supported on the silica gel was about 19% (calculated by supposing that the cellulose derivative, in which 2.5 hydroxyl groups, among 3, contained in the glucose unit of cellulose had been carbamoylated, was supported on the silica gel). Comparative Example 1 (Preparation of separating agent composed of polysaccharide derivative immobilized on silica gel through crosslinkage of both polysaccharide derivative molecules among themselves and polysaccharide derivative with silica gel) 1.8 g of tritylcellulose having 0.9 to 1 trityl group per glucose unit was dissolved in tetrahydrofuran. The solution thus obtained was uniformly poured onto 6.0 g of aminopropylsilane-treated silica gel (mfd. by Daiso Co., Ltd.). The solvent was volatilized and thus the tritylcellulose was supported on the silica gel. Onto this silica gel were poured 75 ml of methanol and 0.75 ml of conc. hydrochloric acid. The obtained mixture was allowed to stand at room temperature overnight to thereby eliminate trityl groups from the cellulose. The product was separated by filtration and washed with methanol. 75 ml of methanol and 0.75 ml of triethylamine were poured thereonto and the resulting mixture was stirred for 5 minutes. The product was separated by filtration, washed with methanol, and then dried. Under a nitrogen atmosphere, one obtained by dissolving 49.3 mg of 4,4'-diphenylmethane diisocyanate in 6.5 ml of dry toluene was added to 3.4 g of the silica gel having cellulose adsorbed thereon as obtained above. Further, 2.5 ml of dry pyridine was added thereto. The mixture thus obtained was stirred at 60°C under heating. After 5 hours, 20 ml of dry pyridine and 0.75 ml of 3,5-dimethylphenyl isocyanate were added thereto, followed by heating to 110°C. After 18 hours, the reaction mixture thus obtained was filtered through a glass filter and the precipitate thus obtained was washed with tetrahydrofuran. The precipitate was washed successively with methanol, ethanol and n-hexane before drying, and then dried. Thus was obtained a separating agent composed of the polysaccharide derivative immobilized on the silica gel through crosslinkage of both of the polysaccharide derivative molecules among themselves and the polysaccharide derivative with the silica gel. The amount of the cellulose derivative supported on the silica gel was about 18% (calculated by supposing that the cellulose derivative, in which 2.5 hydroxyl groups, among 3, contained in the glucose unit of cellulose had been carbamoylated, was supported on the silica gel). Comparative Example 2 (Preparation of separating agent composed of silica gel coated with cellulose tris(3,5-dimethylphenylcarbamate)) Under a nitrogen atmosphere, 3.5 kg of cellulose (degree of polymerization about 300) was added to 56 l of pyridine. Then 3,5-dimethylphenyl isocyanate was added thereto in large excess (23.1 kg) of the cellulose at 100°C. The mixture thus obtained was stirred at 105°C and reacted for 12 hours. Next, the reaction liquid was cooled and 3 l of methanol was added thereto. The obtained mixture was poured into 160 l of methanol. The precipitate thus formed was recovered by filtration and then dried. Thus 11.8 kg of cellulose tris(3,5-dimethylphenylcarbamate) was obtained (yield 88%). 720 g of the cellulose tris(3,5-dimethylphenylcarbamate) thus obtained was dissolved in 4.7 l of acetone. The resulting solution was dropped into 2,880 g of 3-aminopropylsilane-treated silica gel (Daiso SP-1000) under stirring. The mixture thus obtained was completely mixed and then the solvent was volatilized. Thus 3,580 g of a separating agent was obtained. Application Example 1 The separating agent composed of a polysaccharide derivative immobilized on silica gel prepared in Example 1 was employed as a column packing. It was packed in a stainless steel column having a height of 25 cm in height and an inner diameter of 0.46 cm by the slurry packing method to thereby prepare a column for optical resolution. This column was used as such or after washing with various organic solvents. The optical resolution of various racemic modifications as shown in Table 1 was effected. As the high performance liquid chromatography (HPLC), JASCO 875-UV, mfd. by Nippon Bunko, was used. The experiment was conducted at a flow rate of the eluent of 1.0 ml/min and at a temperature of 25°C. Table 1 shows the results. The terms given in the table are each defined as follows. Capacity factor (k 1 ') = (retention time of antipode) - (dead time) (dead time) Separation factor (α) = capacity factor of antipode adsorbed more strongly capacity factor of antipode adsorbed more weakly Resolution (Rs) = 2 x distance between peak of antipode adsorbed more strongly and peak of antipode adsorbed more weakly sum of band widths of both peaks Application Example 2 The separating agent prepared in Comparative Example 1 was used as a column packing and packed into a stainless column having a height of 10 cm and an inner diameter of 0.46 cm by the slurry packing method to thereby prepare a column. After the column thus obtained was washed with various organic solvents, the optical resolution of various racemic modifications as shown in Table 1 was effected with the use of the column. As the high performance liquid chromatography (HPLC), JASCO 875-UV, mfd. by Nippon Bunko, was used. The experiment was conducted at a flow rate of the eluent of 0.4 ml/min and at a temperature of 25°C. Table 1 shows the results. Application Example 3 The separating agent prepared in Comparative Example 2 was used as a column packing and packed into 2 stainless columns each having a height of 25 cm and an inner diameter of 0.46 cm by the slurry packing method to thereby prepare 2 columns for optical resolution. One column as such was subjected to the experiment for optical resolution of various racemic modifications as shown in Table 2 and thus the results as given in Table 2 were obtained. The experiment was conducted under conditions that the eluent was a mixture of hexane with isopropanol (9/1 (v/v)), the flow rate of the eluent was 1.0 ml/min and the temperature was 25°C. Another column was washed successively with 4 mixtures of hexane/isopropanol/tetrahydrofuran (mixing ratio (volume ratio) = 9/1/1, 9/1/2, 9/1/4 and 9/1/8) and methanol each for 30 minutes and then subjected to the experiment for optical resolution of various racemic modifications as shown in Table 2. Thus the results as given in Table 2 were obtained. The experiment was conducted under conditions that the eluent was a mixture of hexane with isopropanol (9/1 (v/v)), the flow rate of the eluent was 1.0 ml/min and the temperature was 25°C.

Claims: A separating agent for optical isomers characterized by being composed of a polysaccharide derivative which has been immobilized on a support by crosslinking, on said support, the polysaccharide derivative molecules with the use of a polyfunctional crosslinking agent which does not form a link between said support and said polysaccharide derivative molecules, and wherein the surface of said support has been inactivated prior to said crosslinking reaction in order to prevent the surface of said support from reacting with said crosslinking agent. The separating agent for optical isomers as set forth in Claim 1 wherein the support is a surface-inactivated silica gel having a particle size of from 1 µm to 10 mm and a pore size of from 10 to 100 µm. The separating agent for optical isomers as set forth in Claim 1 wherein the polysaccharide derivative before the crosslinkage is an ester or a carbamate derivative of cellulose or amylose having at least 0.1 unreacted hydroxyl group per glucose unit. The separating agent for optical isomers as set forth in Claim 1 wherein the polysaccharide derivative before the crosslinkage is an ester or a carbamate derivative of cellulose or amylose having at least 0.1 reactive functional group per glucose unit in the groups introduced into cellulose or amylose so as to convert it into the ester or carbamate derivative thereof. The separating agent for optical isomers as set forth in Claim 1 which is used in chromatography. A process for producing a separating agent for optical isomers as set forth in Claim 1 characterized by crosslinking, on a support, polysaccharide derivative molecules with the use of a polyfunctional crosslinking agent which does not form a link between said support and said polysaccharide derivative molecules, wherein the surface of said support has been inactivated prior to said crosslinking reaction in order to prevent the surface of said support from reacting with said crosslinking agent. The process for producing a separating agent for optical isomers as set forth in Claim 6 wherein the polysaccharide derivative before the crosslinkage is an ester or a carbamate derivative of cellulose or amylose having at least 0.1 unreacted hydroxyl group per glucose unit. The process for producing a separating agent for optical isomers as set forth in Claim 6 wherein the polysaccharide derivative before the crosslinkage is an ester or a carbamate derivative of cellulose or amylose having at least 0.1 reactive functional group per glucose unit in the groups introduced into cellulose or amylose so as to convert it into the ester or carbamate derivative thereof. The process for producing a separating agent for optical isomers as set forth in Claim 6 wherein said polyfunctional crosslinking agent is a diisocyanate derivative, an acid chloride derivative of a dicarboxylic acid, a diepoxy derivative or a divinyl derivative. The process for producing a separating agent for optical isomers as set forth in Claim 6 wherein the hydroxyl groups at the 6-position of cellulose or amylose are selectively crosslinked with each other. A use of a separating agent for optical isomers as set forth in Claim 1 as a column packing in chromatography. A process for separating a mixture of optical isomers which comprises using a separating agent for optical isomers as set forth in Claim 1 to thereby separate the mixture of optical isomers from each other.

Inventors: ICHIDA AKITO, MURAKAMI TATSUSHI, ICHIDA, AKITO, MURAKAMI, TATSUSHI
Applicants: DAICEL CHEM, DAICEL CHEMICAL INDUSTRIES, LTD.
Application year: 1994
Main classification: C07B57
IPC codes: B01J20, C07B57, C08B37, G01N30